T lymphocytes must respond to specific antigens by rapid proliferation and differentiation to mount an effective immune response. It is critical that this occur only in response to foreign antigen, so that self antigens do not induce autoimmune responses. T cell tolerance to self-antigen is achieved in part by negative selection in the thymus to eliminate clones that recognize self-antigen. This is not complete, however, and some self-reactive T cells escape into the periphery. Mechanisms exist for rendering these mature T cells tolerant to self antigens, but they are poorly understood. This Program is addressing the nature of these mechanisms in mature T cells at the molecular and cellular levels using both in vitro and in vivo models. It is anticipated that the findings obtained in the planned studies will contribute to a better fundamental understanding of how autoimmunity is avoided. In addition, understanding of these mechanisms, and hence the ability to manipulate them, has the potential to contribute to improvements in transplantation and disease therapy. Mechanisms that induce tolerance to self-antigens may also induce tolerance to foreign antigens including those present on tumors or virus-infected cells, resulting in the immune system failing to mount a protective response. Finally, there is great potential for using defined peptide antigens to induce protective or therapeutic immunity for a broad range of diseases and much current effort is focusing on this. However, it is becoming increasingly clear that these must be used with great caution since they can also induce tolerance that may lead to lessened protection or exacerbated disease. Thus, developing a better understanding of the mechanisms that can lead to T cell tolerance, as proposed in this Program, has implications well beyond autoimmune diseases.